The demand for improved operational amplifiers, and in particular amplifier circuits for high-precision data acquisition and instrumentation applications, such as multi-channel data acquisition systems, current shunt monitors, and industrial or physiological sensors, continues to increase. High precision amplifier circuits typically require at least 0.1% accuracy with input full-scale signals as low as 5 mV. Critical specifications for such devices include the amount of offset voltage, the amount of offset drift, and the peak-to-peak noise.
One approach for minimizing offset voltage in an operational amplifier, such as an instrumentation amplifier, is through implementation of auto-zeroing techniques. In such auto-zeroing techniques, one or more Gm stages in the operational amplifier are configured within an auto-zero loop to generate an offset correcting voltage comprising an output-referred offset error opposite in magnitude to the offset voltage of the operational amplifier. The offset correcting voltage is summed with the offset voltage to result in no offset effect in the operational amplifier, i.e., the offset voltage does not translate through to the output of the instrumentation amplifier.
With reference to FIG. 1, a sensor amplifier circuit 100 is illustrated, with amplifier circuit 100 including a sensor 102 and an instrumentation amplifier 104. Sensor 102 is represented by a bridge circuit comprising four resistors R, and can comprise any sensor circuit providing a sensed voltage to instrumentation amplifier 104. Instrumentation amplifier 104 comprises a pair of operational amplifiers A1 and A2 configured in a differential input/differential output configuration, i.e., with input terminals VIN+ and VIN− and output terminals VOUT+ and VOUT−, and can also comprise various other instrumentation amplifier configurations. Operational amplifiers A1 and A2 can be configured with auto-zero configurations of one or more stages to correct for the offset voltage within instrumentation amplifier 104.
Some input source devices, such as sensors, include a useful small signal riding on top of a relatively large offset voltage. The offset voltage needs to be subtracted before the small signal is provided to an amplifier to prevent over-ranging of the signal path. Unfortunately with present auto-zeroing techniques, an offset voltage VOSSENSOR of sensor 102 is not corrected, i.e., only the offset voltage of instrumentation amplifier 104 is corrected, and thus offset voltage VOSSENSOR can translate, or is gained up, by instrumentation amplifier 104. For example, for offset voltage VOSSENSOR equal to approximately 10 mV, and with instrumentation amplifier 104 having a gain of approximately 100, an offset error of approximately 1V can result at output terminals VOUT+ and VOUT− of instrumentation amplifier 104. Accordingly, the dynamic range at the output of amplifier 104 is lost due to the translating of offset voltage VOSSENSOR.
To minimize problems caused by offset voltage VOSSENSOR, manufacturing techniques can be applied, such as laser trimming, while other techniques can include processing offset voltage VOSSENSOR and then subtracting out the translated signal. Unfortunately, such techniques are more complicated or costly, e.g., the addition of trimming procedures to the manufacturing process, or the requirement to amplify and process offset voltage VOSSENSOR before the offset is subtracted, thus reducing the dynamic range, and potentially saturating instrumentation amplifier 104.
Another approach to reducing the offset within an operational amplifier includes the use of long-used potentiometer devices to address the offset error through manual adjustment of the offset voltage of the operational amplifier in a continuous-time manner. However, such potentiometer techniques are not adequate for high-speed, high-precision applications currently being demanded. Other approaches utilize sampled switch-capacitor techniques, but unfortunately these switch-capacitor techniques cannot be readily adopted for continuous-time signal applications. Moreover, the above techniques still translate offset error with any input source, such as sensors, to the operational amplifier output at the cost of dynamic range performance.